Design of a two-way car alarm based on LPC930

0 Introduction

Cars are currently the main means of transportation for humans and a symbol of modern civilization. The annual sales of cars in the world reach more than 60 million, and the number of cars in use has exceeded 500 million. The more cars are used, the more cars are stolen. Therefore, car theft prevention has become an important social issue. It has been listed as one of the four major topics of automobile technology development along with safety, environmental protection and energy saving.

According to their structure and function, automobile anti-theft devices can be divided into three categories: mechanical, network and electronic. The principle of mechanical anti-theft is to lock a certain structure on the car with a mechanical lock, such as the transmission, steering wheel, etc. This type of anti-theft device is easy to install and cheap, but it is large in size, and this type of anti-theft device only prevents theft but does not alarm, and cannot ensure anti-theft. Network anti-theft mainly relies on the public network of society to monitor the driving of vehicles, such as GPS positioning system, GSM or GPRS, etc. This type of anti-theft device has advanced technology and powerful functions, but it is expensive and requires service fees. In addition, the communication signal is easily interfered, which reduces the anti-theft performance. Electronic anti-theft devices are the most popular anti-theft devices in the current automobile market. The radio transmitter chip in the key can communicate with the ECU in the car body to achieve one-way or even two-way alarms. When the car is invaded by the outside world, the owner nearby can understand the condition of the car through the display screen on the key he carries with him, but its disadvantage is that the false alarm rate is high.

This paper presents a two-way electronic car anti-theft system based on a single-chip microcomputer, which consists of a host and a remote control. The remote control is carried by the car owner, and the host is placed in the car to detect the alarm signal source. A custom wireless transceiver module is used to perform half-duplex communication between the two.

1 Host Hardware Design

The host is placed in the car and consists of five modules: MCU, power supply, sensor input, high-frequency module, and alarm output. The system block diagram is shown in Figure 1. The main controller MCU uses LPC930, which is used to detect the trigger of the sensor and generate an alarm signal; at the same time, the status of the remote control is updated to achieve synchronous alarm. Since the sensor input module requires a DC voltage of 12 V, and the operating voltage of LPC930 is between 2.4 and 3.6 V, the SPX1117 regulator is used in the power module to generate a DC voltage of 3.3 V. The sound alarm control circuit uses the RT0100 circuit. RT0100 is a crystal circuit that can generate a single alarm sound. It is manufactured using CMOS technology and has a built-in RC oscillation circuit. The operating voltage is 2 to 5 V and the static current is low.

1.1 Sensor module design

The sensor module includes a side door detection circuit and a vibration detection circuit. When various alarms are triggered, the car horn will sound for 30 seconds, the turn signal will flash, and the vehicle will be turned off and cannot be started. After the alarm is completed, the anti-theft system automatically returns to the state before the alarm. If the same sensor is detected to be triggered continuously in a short period of time, the anti-theft system will only alarm for 4 minutes and then automatically stop the alarm. Until other sensors are triggered, all detection points will be re-detected. At this time, the same sensor is still detected to be triggered, and the alarm will be again for 4 minutes.

1.1.1 Side door detection circuit

When the host is in the alert state, the side door detection circuit is used to detect whether the side door is opened. If the side door is opened for no reason, the host enters the alarm state. The detection circuit is shown in Figure 2. Point A in the figure is connected to the side door, and point B is connected to the single-chip microcomputer. When the side door is closed, point B is charged to a high level due to the reverse cutoff of diode D2. When the side door is opened, point A becomes a low level, diode D2 is turned on, the relay switch is grounded, and the RC circuit composed of C1 and R2 is quickly discharged. Point B is pulled to a low level, generating a low level signal to the single-chip microcomputer, and the single-chip microcomputer controls the alarm output circuit to alarm.

C2 is used to filter low-level glitch pulses to prevent the system from malfunctioning. Diode D1 and the relay coil form a discharge circuit. When the side door is closed, the residual charge is discharged through D1 due to the inductance of the relay coil.

1.1.2 Vibration detection circuit

When the host is in the alert state, the vibration detection circuit is used to detect whether the external interference causes damage to the vehicle body. If the vehicle body vibration caused by the external interference exceeds the limit that the vehicle body can bear, the host enters the alarm state. The circuit diagram is shown in Figure 3, where point A is connected to the vibration detection sensor and point B is connected to the microcontroller. When vibration is detected, A becomes low level, D1 is turned on, and the RC circuit composed of C1 and R2 is discharged quickly through D1, causing point B to quickly become low level. The voltage across C1 cannot jump, so this feature is used to filter the low-level glitch pulse generated by the vibration to ensure accurate detection of the vibration jump signal.

1.2 Alarm output module

In addition to the simple sound and light alarms of the horn and lights, the alarm output module also uses a flameout output control circuit. When the host is in the alarm state, the vehicle is turned off and cannot be started. The circuit diagram is shown in Figure 4, where point A is connected to the flameout output controller and point B is connected to the single-chip microcomputer. When the single-chip microcomputer outputs a high level, the transistor Q1 is turned on, point A becomes a high level, and a flameout output signal is generated, and the car cannot be started. The transistor Q2 plays a shunt protection role. When the emitter current of the transistor Q1 exceeds the upper limit, Q2 will automatically turn on to prevent Q1 from being damaged due to overcurrent. The varistor RU1 protects Q1 and Q2. When the collector voltage of Q1 does not exceed the upper limit, RU1 will not turn on, but when the collector of Q1 is overvoltage, RU1 will automatically turn on to prevent Q1 from being damaged during overvoltage.

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2 Remote Control Hardware Design

The remote control part consists of MCU, wireless transceiver module, power supply, keyboard, and alarm module. LPC930 is used as the core device to control the operation of the surrounding modules. The wireless transceiver module with TDA5255 as the core receives the data from the host and forwards it to LPC930 for processing. The system block diagram of the remote control part is shown in Figure 5.

The wireless transceiver module consists of four parts: antenna, high-frequency transmission, high-frequency reception, and TDA5255. The TDA5255 chip is a powerful, low-power FSK/ASK single-chip transceiver chip produced by Infineon, Germany. It works in the 433-435 MHz frequency band and has FSK/ASK modulation and demodulation functions. It has a high degree of integration and has complete VCO (voltage-controlled oscillator) and PLL (phase-locked loop) synthesizers, FSK modulators, RSSI limiters, FSK demodulators, data filters, data dividers, etc., which reduces the design of peripheral circuits. More importantly, the chip has a power-saving mode function, which can be set in different ways to meet the low-power requirements of the remote control. The system block diagram of the module is shown in Figure 6.

3 System Software Design

3.1 Host software design

The software part of the host mainly includes four parts: wireless data transmission, data processing and feedback, sensor detection, alarm output and feedback. There are more than 20 states and functions to be processed, and it must be real-time and have good interactivity with the remote control. Two variables STATUS and D-STATUS are used to store the status of the system and alarm respectively, and jump according to the status. The overall flow chart is shown in Figure 7.

3.2 Remote Control Software Design

The remote control software design takes keystroke as the first response and data reception as the second response. In addition to functions such as data transmission, alarm, key arming and disarming, it also needs additional functions such as music generation, low voltage detection, low power consumption control, and signal strength detection. The software design flow chart is shown in Figure 8.

4 Conclusion

This system uses a single-chip microcomputer as the main control, and adopts a custom communication protocol to achieve half-duplex communication between the host and the remote control, thereby achieving the function of anti-theft alarm. The remote control controls the status of the host and can alarm synchronously with the host; the host can detect multiple trigger sources to realize remote alarms for the host itself and the remote control, and can complete the functions set by the remote control such as automatic door locking, automatic alarm recovery, and emergency alarm release. After testing, this system has the characteristics of strong real-time performance, high reliability, and low power consumption.